The problems involved in the measurement of liquid volume in tanks, and especially in tanks carrying corrosive liquids, are considerable. These problems are emphasized in fuel tanks of modern jet aircraft, which undergo extreme changes in temperature, pressure, and attitude.
For example, many aircraft utilize corrosive fuels such as hydrazine-water blends. Such corrosive liquids may be volatile and incompatible with conventional construction materials such as copper, plastic, etc. Therefore, not only must the tank be of stainless steel or another corrosion-resistant material, but any auxiliary device, such as a fuel quantity indicator, must be equally resistant to corrosion.
The measurable volume of liquid fuel in an aircraft tank is constantly changing, mainly due to consumption. Constant changes in volume are also caused by expansion and contraction of the liquid in response to temperature and pressure changes caused by a change in altitude, for example. Fuel quantity indicators must be responsive to even small changes in liquid volume.
It is also important that liquid volume indicators be durable and reliable, with few moving parts which, if worn, may cause early failure.